In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, 3rd Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible implementations. A radio communications network comprises radio network nodes providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. User equipments (UE) are served in the cells by the respective radio network node and are communicating with respective radio network node. The user equipments transmit data over an air or radio interface to the radio network nodes in uplink (UL) transmissions and the radio network nodes transmit data over an air or radio interface to the user equipments in downlink (DL) transmissions.
Traditional radio communications networks broadcast cell-specific reference signals and system information. These signals let user equipments determine which cell the user equipments should connect to and provide information to the user equipments how they should access those cells. Radio network nodes that are adjacent or close to each other need to transmit different reference signals, so that the user equipments can distinguish between them, and determine which cell or cells the user equipments should connect to.
In particular, the broadcasted system information comprises parameters that control the timing, frequency, transmission formats, and power used by the user equipment for initial (random access) transmissions to the network. Such information may be different in different cells, e.g. to be able to distinguish between accesses made in different cells, or to adjust the initial user equipment transmission power levels so as to fit the characteristics of different cells. The user equipment typically determines its initial transmission power using a standardized formula that comprises as a part the received power of the cell-specific reference signal as measured by the user equipment, and may also comprise one or more parameters that are related to the transmission power of the radio network node. In this way, the transmission power of the signal initially transmitted by a user equipment performing random access is adjusted such that it is likely received by a selected radio network node with a desired level: high enough for the signal to be detectable, but not so high that the signal interferes too much with other signals in the radio communications network.
Since the radio communications network, i.e. the radio network node, does not know the location or presence of the user equipments, the cell-specific signals are transmitted with constant and relatively high power and high periodicity. This is to ensure that all user equipments can read these signals at all times.
Future radio communications networks will be denser, having more access nodes than today's network nodes. In some scenarios, the number of access nodes may be considerably larger than the number of user equipments. These nodes may be more coordinated than traditional cells, for instance they may be implemented as remote radio heads, connected with a high bit-rate backhaul link to a network node in the core network.
With dense radio communications networks, it would be very costly to broadcast different reference signals and system information from each radio network node, also referred to as access node, because of the large number of radio network nodes. Further, at any time instant, most of these radio network nodes would not have any user equipments, thus making such transmissions unnecessary in practice. Also, a moving user equipment may move between access nodes more often, compared to traditional cells, making it more cumbersome for the user equipment to track the reference signals and read the system information from each access node.
The energy consumption of a radio communications network with a much denser deployment than today would become unacceptably high if all radio network nodes where to transmit individual system information. Also, the interference level would always be rather high in such a radio communications network due to system information pollution and hence, even at very low traffic the Signal to Noise plus Interference Ratio (SINR) will never become really high.
The obvious solution is then to conclude that individual radio network nodes in a future dense or a super-dense deployment should not transmit individual system information. The problem that arises then is that the user equipment will not be able to obtain information on how to access the system. One problem is the uplink power setting, as indicated in FIG. 1, where high power is required to reach a radio network node A, medium power is required to reach a radio network node B, and low power is required to reach a radio network node C. Even if it is assumed that it can be afforded to transmit some low duty-cycle downlink reference signal from each radio network node for the user equipment to measure on, the user equipment will not have any information on the power that the reference signals are transmitted with. Without knowing the transmitted power the user equipment cannot estimate the path-loss to each respective radio network node and hence it cannot perform an initial access transmission with an appropriate power level. The power level is only one problem that the user equipment cannot solve. Without system information the user equipment cannot determine the uplink frequency band to use, which pre-amble to use to access the radio communications network, how to handle Random Access Channel (RACH) congestion, etc.
It is worth noting that it is not always a problem if the user equipment transmits with unnecessarily high power when accessing a nearby radio network node. In a first scenario, illustrated in the left part of FIG. 2, a user equipment that is positioned close to a micro radio network node is transmitting with high power, calculated to be sufficient to reach a macro radio network node located far away. In case there is no active transmission in the micro cell then no on-going communications gets disturbed by this interference. In another scenario, illustrated in the right part of FIG. 2, however there is an on-going uplink transmission between the micro radio network node and another user equipment. In this case the on-going communication could be severely interfered in case a user equipment performs an initial access with far too high transmission power.
Thus, transmitting system information from each radio network node, as done today in state-of-the-art, is not energy efficient for dense deployments. Furthermore, the resulting interference from pilot pollution and system information pollution reduce SINR when load is low.